I do not know if it is just my portal which is missing the “Enable Device Management PREVIEW” – Checkbox when creating a new IOT-Hub. It is still described in the article “Tutorial: Get started with device management” from Juan Perez .
You still can create it with an ARM Template by setting the “Feature”-Field to “DeviceManagement”.
I have written an ARM Template for an IOT-Hub with Device Management which you can download here. Be aware though, that this only works in certain regions like NorthEurope (but now WestEurope).